This invention generally relates to the electrical connection of fuel cell stacks and, more particularly, to the series and parallel electrical connection of fuel cells, such as oxygen-ion conducting solid oxide fuel cells and proton conducting ceramic or polymer membrane fuel cells, in which the electrolyte is a solid.
A fuel cell is basically a galvanic conversion device that electrochemically reacts a fuel with an oxidant to generate a direct current. A fuel cell typically includes a cathode material, an electrolyte material, and an anode material. The electrolyte is a non-porous material sandwiched between the cathode and anode materials. An individual electrochemical cell usually generates a relatively small voltage. Thus, to achieve higher voltages that are practically useful, the individual electrochemical cells are connected together in series to form a stack. To achieve a desired current, individual cells are connected in parallel. Electrical connection between cells is achieved by the use of an electrical interconnect between the cathode and anode of adjacent cells. The electrical interconnect also provides for passageways which allow oxidant fluid to flow past the cathode and fuel fluid to flow past the anode, while keeping these fluids separated. Also typically included in the stack are ducts or manifolding to conduct the fuel and oxidant into and out of the stack.
The fuel and oxidant fluids are typically gases and are continuously passed through separate passageways. Electrochemical conversion occurs at or near the three-phase boundaries of each electrode (cathode and anode) and the electrolyte. The fuel is electrochemically reacted with the oxidant to produce a DC electrical output. The anode or fuel electrode enhances the rate at which electrochemical reactions occur on the fuel side. The cathode or oxidant electrode functions similarly on the oxidant side.
Fuel cells with solid electrolytes are the most promising technologies for power generation. Solid electrolytes are either ion conducting ceramic or polymer membranes. In the former instance, the electrolyte is typically made of a ceramic, such as dense yttria-stabilized zirconia (YSZ) ceramic, that is a nonconductor of electrons, which ensures that the electrons must pass through the external circuit to do useful work. With such an electrolyte, the anode is oftentimes made of nickel/YSZ cermet and the cathode is oftentimes made of doped lanthanum manganite.
Perhaps the most advanced construction with ceramic membranes is the tubular solid oxide fuel cell based on zirconia. The tubular construction can be assembled into relatively large units without seals and this is its biggest engineering advantage. However, tubular solid oxide fuel cells are fabricated by electrochemical vapor deposition processes, which are slow and costly. The tubular geometry of these fuel cells also limits the specific power density, both on weight and volume bases, to low values. The electron conduction paths are also long and lead to high energy losses due to internal resistance heating. For these reasons, other constructions are actively being pursued.
One common alternative construction to the tubular construction is a planar construction that resembles a cross-flow heat exchanger in a cubic configuration. The planar cross flow fuel cell is built from alternating flat single cell membranes (which are trilayer anode/electrolyte/cathode structures) and bipolar plates (which conduct current from cell to cell and provide channels for gas flow into a cubic structure or stack). The bipolar plates are oftentimes made of suitable metallic materials. The cross-flow stack is manifolded externally on four faces for fuel and oxidant gas management.
Another alternative construction to the tubular design is a radial or co-flow design. An annular shaped anode and cathode sandwich an electrolyte therebetween. Annular shaped separator plates sandwich the combination of anode, cathode, and electrolyte. A fuel manifold and an oxidant manifold respectively direct fuel and oxidant to a central portion of the stack so that the fuel and oxidant can flow radially outward from the manifolds.
Nevertheless, in either the radial or cross-flow fuel cell stack designs, electrical connection between fuel cell stacks remains an issue. In particular, both designs impose a series electrical connection between stacks, while making a parallel electrical connection difficult. Further, the current art of connecting one stack to another leads to the disadvantage of rendering all of the connected stacks inoperable if one cell or stack becomes inoperable. Additionally, an electrical connection between stacks can result in an increased size of the overall dimensions of the connected stacks. Examples of various electrical connections of fuel cell stacks are found in U.S. Pat. Nos. 5,874,183; 5,750,279; 5,258,240; and 5,034,288.
As can be seen, there is a need for an improved apparatus and method of electrical connection for fuel stacks having solid electrolytes. In particular, there is a need for a parallel electrical connection between fuel stacks having either a cross-flow design or a radial flow design. An apparatus and method are needed that allows an electrical connection among the stacks, even in the event that one cell or stack becomes inoperable. Also needed is an apparatus and method that maintains the benefit of a reduced size for a cross-flow or radial flow stack, while still providing a parallel electrical connection.
Accordingly, an object of the present invention is to provide an electrical connector between the current conducting plates of fuel cell stacks having solid electrolytes.
Another object of the present invention is to provide a parallel electrical connector for cross-flow and radial flow fuel cell stacks at the xe2x80x9ccell levelxe2x80x9d or at a xe2x80x9chigher cell level.xe2x80x9d The xe2x80x9ccell levelxe2x80x9d is defined as having electrical connection between each cell of both stacks. xe2x80x9cHigher cell levelxe2x80x9d is defined as a set of two or more unconnected cells that intervene between connected cells of adjacent stacks. In other words, not all of the cells are connected in parallel.
Yet another object of the present invention is to provide a simple and cost-efficient apparatus and method of electrically connecting in parallel two or more fuel cell stacks at the cell or higher cell level.
An additional object of the present invention is to provide high power density of fuel cell stacks, while allowing series and parallel electrical connection among the stacks at the cell or higher cell level.
A further object of the present invention is to provide a parallel electrical connection among fuel cell stacks to circumvent the possibility that one or more cells of a stack become inoperable.
The present invention achieves the foregoing objects, as well as others, by a parallel electrical connector between a first conducting plate of a first fuel cell stack and a second conducting plate of a second fuel cell stack, comprising a connector element affixed to the first and second conducting plates. The connector element is adjacent a first open face of the first fuel cell stack and a second open face of the second fuel cell stack. The connector is preferably made of the same material as the conducting plates. Also, the connector element is positioned substantially parallel to at least one of the first and second conducting plates.